Physics 202 MWF 10:20 Spring 1997 (Ford)

Exam I Chapts. 24-27 in SZY College Physics 7e

Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.

(14 pts) 1. A particle with charge q and mass m = 0.050 kg is placed in a uniform electric field that is directed downward and that has magnitude E = 8.0 × 104 N/C. What is the sign and magnitude of q so that the particle will remain stationary if released from rest?

Ans. sign ___negative___

magnitude ___6.12 × 10-6 C___


(20 pts) 2. Point charge q1 = +5.0 × 10-6 C is on the +y-axis at y = +0.1 m and point charge q2 = -5.0 × 10-6 C is on the -y-axis at y = -0.1 m. Point P is on the +x-axis at x = +0.2 m.

a) What are the x and y components of the resultant electric field produced at point P by the two point charges q1 and q2?

Ans. Ex = ___0___

Ey = ___-8.1 × 105 N/C___

b) A point charge q3 = -3.0 × 10-6 C is placed at P. What are the magnitude and direction of the resultant force exerted on q3 by the two charges q1 and q2?

Ans. F = ___2.4 N___

direction ___+y___


(16 pts) 3. A small solid conducting sphere with radius a is concentric with a larger conducting spherical shell with inner radius b and outer radius c. The electric field for a < r < b (in the air space between the sphere and the shell) has magnitude E = 3q/ 40r 2 and is directed in toward the center of the sphere. The electric field for r > c (outside the spherical shell) has magnitude E = 4q/ 40r 2 and is directed radially outward. The parameter q represents a positive quantity of charge.

a) What is the electric field in the region b < r < c (inside the metal of the shell)?

Ans. ___0___

b) In terms of q, what is the net charge on the solid sphere? Is the net charge positive or negative? (To receive credit you must give the reasoning behind your answer.)

Ans. ___-3q___

c) In terms of q, what is the net charge on the spherical shell? Is the net charge positive or negative? (To receive credit you must give the reasoning behind your answer.)

Ans. ___+7q___


(16 pts) 4. A parallel-plate capacitor has charge Q = 9.0 × 10-6 C and capacitance C0 = 3.0 × 10-10 F when there is air between the plates. Each plate has an area of 0.14 m2. With the charge on the plates of the capacitor kept constant, a dielectric with K = 5 is inserted between the plates, completely filling the volume between the plates. After the dielectric has been inserted,

a) what is the potential difference between the plates of the capacitor?

Ans. ___6000 V___

b) what is the magnitude of the electric field at a point midway between the plates?

Ans. ___1.45 × 106 V/m___


(16 pts) 5. In the circuit shown in the sketch C2 = 3.0 µF. The potential difference Vab applied to the capacitor network is 120 V. The charge on the capacitor C2 is Q2 = 1.5 × 10-4 C. Calculate the voltage V1 for the capacitor C1 and the voltage V3 for the capacitor C3.

V1 ___50 V___

V3 ___70 V___


(18 pts) 6. A small, charged metal sphere, carrying a net charge of q1 = +6.00 × 10-6 C, is held in a stationary position by insulating supports. A second small metal sphere, with a net charge of q2 = +3.00 × 10-6 C and a mass of 5.00 g, is projected toward q1. When the two spheres are 0.800 m apart, q2 is moving toward q1 with a speed of 12.0 m/s. Assume that the two spheres can be treated as point charges and neglect the effects of gravity. How close does q2 get to q1?

Ans. ___0.288 m___